Since carbon nanofiber are nano-level fine carbon fibers and have excellent conductivity, the carbon nanofiber have been widely used as a conductive material and &so widely used as a functional material based on mechanical properties or the like.
Carbon nanotubes can be produced by an electrode discharge method, a vapor phase growth method, a laser method and the like. Among them, in the vapor phase growth method, for example, oxides of iron, nickel, cobalt, magnesium and the like are used as a catalyst, and a mixed gas of either one of carbon monoxide or carbon dioxide and hydrogen is used as a raw material gas. Then, the raw material gas is subjected to thermal decomposition at a high temperature to generate carbon, and the generated carbon is grown in a fiber shape using a catalyst particle as a nucleus (Patent Documents 1 and 2).
In a production method by vapor phase growth in which a catalyst is used, when a raw material gas is subjected to a vapor phase reaction (thermal decomposition), a metal oxide as a catalyst component is reduced by hydrogen in the raw material gas to generate metal, a carbon-containing metal, or a metal-carbon compound. This product adheres to and remains on the carbon nanofibers in some cases. The physical properties of the fiber may be damaged depending on the type of metal remaining on the surface of the fiber.
For example, in a production method by vapor phase growth in the related art, it has been known that a mixed powder of CoO and MgO, or a mixed powder of Co3O4 and MgO is used as a catalyst. When a raw material gas is subjected to thermal decomposition, at least one type of metal-based Co selected from metal cobalt, carbon-containing cobalt metals, and cobalt-carbon compounds is generated. If the metal-based Co remains on the surfaces of fibers in an unstable state, in the case where carbon nanofibers are used as a conductive filler in a resin composition, or as a conductive assistant in a lithium secondary battery of the like, electric properties such as conductivity, strength, lifespan and the like are affected by the metal-based Co.
In the related art, generally, carbon nanofibers produced by vapor phase growth in which a catalyst is used are immersed in hydrochloric acid nitric acid, hydrofluoric acid, or the like so as to remove a residual catalyst. In addition, it has been known that the carbon nanofiber are used in a state where catalyst particles are included in the carbon nanofibers as they are and the catalyst particles are supported by the carbon nanofibers (Patent Document 1). However, the metal-based Co which is included in the carbon nanofibers and is supported by the carbon nanofibers is not necessarily stable. Thus, if the metal-based Co is detached or exfoliated from the fibers, electric properties, strength, durability or the like of a product using the carbon nanofibers are affected.